1. Field of the Invention
This invention relates to an image forming apparatus including a fixing unit used for electrophotographic photocopier, laser beam printer, and the like.
2. Description of Related Art
As a fixing unit for image forming apparatuses such as photocopiers and printers, units of thermal roller types and on-demand types have been known.
A fixing unit of the thermal roller type includes an internal thermal source such as a halogen heater or the like, and a recording material is introduced to a pressed nipping portion (fixing nipping portion) located between two rollers in a transfer unit having the two rollers, a heating roller (fixing roller) kept at a prescribed heated temperature by means of the thermal source, and an elastic pressing roller pressed to the heating roller. Unfixed images transferred on the recording material with heat from the fixing roller can be fixed on the recording material where the recording material is conveyed in the conveyance direction as sandwiched at the fixing nipping portion.
The fixing roller contacting to the surface of the recording material, between the fixing roller and the elastic pressing roller, has a cylindrical roller whose surface is made of a material of good mold releasing property, and a halogen heater is incorporated inside the roller to heat the toner transferred on the recording material through the cylindrical roller. On the other hand, the pressing roller contacting to the back surface of the recording material is constituted of a core metal and an elastic layer formed on the core metal for properly pressing the toner layer on the recording material.
The fixing unit of the on-demand type (film heating type) solves a problem on the thermal roller type such as large loss in energy, thereby enabling the unit to start quickly and save electric power. This fixing unit solves the problem on thermal loss in the thermal roller type fixing unit by rendering small the thermal capacity of the fixing unit in using a heater such as a ceramic heater and a heat resisting thin film such as polyimide as substitutions of the halogen heater and the fixing roller in the fixing unit of the thermal roller type.
In the fixing unit of the thermal roller type or the on-demand type, an overheating preventive device, or a so-called thermal protector, is generally arranged as a safety device for preventing the fixing unit from overly heated during malfunction of the fixing unit. When the unit becomes at an extraordinarily high temperature state, the thermal protector begins operating to cut off the thermal generating circuit. As a thermal protector, a thermal fuse, thermo-switch, and the like are exemplified.
The thermal protector is made in direct contact with portions of a heating body or fixing roller equivalent to a sheet passing width of the recording material or is secured to the vicinity of the portions. When the temperature of the arranged placed of the thermal protector reaches the prescribed extraordinary high temperature state, the thermal protector begins operating to cut off the thermal generating circuit.
The thermal protector""s position in a longitudinal direction (a direction perpendicular to the conveyance direction) is generally in compliance with the sheet passing standard position of the image forming apparatus. To prevent the thermal protector from malfunctioning due to increased temperature of a non-sheet passing portion of the fixing unit when small size papers such as envelops and business cards pass, the thermal protector is disposed at an area within the minimum sheet passing width with respect to the sheet passing standard position. That is, the thermal protector is disposed near a center sheet passing standard line where the image forming apparatus is of the center-standard type with respect to the longitudinal direction and near an edge sheet passing standard line where the image forming apparatus is of the edge-standard type of the recording material.
However, with the arrangement structure of the conventional thermal protector, printing can be performed while the arrangement region of the thermal protector is made as the non-sheet passing area according to the positional relation of the recording material sensor such as a recording material existence detecting sensor or a recording material passing sensor.
FIG. 8 is a substantially plan view of a conventional image forming apparatus; the sheet passing standard is of the center standard with respect to the longitudinal direction. In the image forming apparatus shown as the prior art, the maximum sheet passing width during the manual feeding is 216 mm LTR size, and the guaranteed minimum passing width is 76.2 mm as the business card size of 3 inchesxc3x975 inches. As for the recording material sensors for always detecting the recording material during the series of the printing operation, provided are, at a manual feeding tray 21 located below the bottom plate 213, a recording material existence sensor Sf1, not shown, detecting as to whether the recording material exists on the manual feeding tray, at a register portion 22 for feeding the recording material P to a transfer portion 23 in synchrony with arrival of the toner image to the transfer position, a top sensor St for detecting reaching timing of the recording material""s front end in the conveyance direction during conveyance of the recording material and for detecting the conveyance length of the recording material, and at a delivery portion 25, a delivery sensor Sd for detecting the delivery timing of the recording material from the fixing portion 24.
When the recording material sensors do not detect any passage of the recording material at prescribed timings during printing operation, the printing operation is controlled to be stopped, and therefore, the recording material has to be passed through all of the recording material sensors to complete the series of the printing operations.
As for the positions of the recording material sensors with respect to the longitudinal direction, the recording material existence sensor Sf1 and the top sensor St are disposed on a line at an area 33 mm right side of the sheet passing standard line A, and the delivery sensor Sd is disposed on the sheet passing standard line A.
The thermal fuse 100 as a thermal protector is in contact with the heater, but the longitudinal length of the contacting portion is 10 mm, whose center of gravity is located at a point 20 mm left side of the sheet passing standard line A.
The minimum sheet passing width of this image forming apparatus is 76.2 mm; with respect to the center standard, ranges 38.1 mm right and left of the sheet passing standard line A constitute the minimum sheet passing width; the recording material sensor and the thermal protector are disposed within the minimum sheet passing width.
With the above structured image forming apparatus, the series of printing operations can be performed at a state that the arranged area of the thermal protector is made of the non-sheet passing region.
That is, as shown in FIG. 8, the recording material width regulation plate 212 mounted on a manual feeding tray 210 is made widest, and the series of printing operations can be performed by passing the recording materials through all the recording material sensors while the arranged are of the thermal protector 100 is made as the non sheet passing area where the recording material such as an envelop having the width of 110 mm is passed as shifted toward the right edge.
Thus, where the sheet passage is possible in a manner of feeding different from the product specification, printing can be done while the arranged area of the thermal protector is made as the non sheet passing area depending on the positional relation of the recording material sensors, so that the temperature is readily increased due to the increase of the temperature of the non sheet passing area, and so that margins for the operation standard temperature of the thermal protector becomes small.
Though not guaranteed, where the recording material width regulation plate 212 is narrowed at minimum to pass the materials, for example, such as stripes having a width of 40 mm at a state that placed toward the right edge, or namely, where the recording materials having a width less than the guaranteed minimum sheet passing width is passed, substantially the same problem may be raised. This is also applicable to an image forming apparatus of the edge sheet passing standard type.
It is an object of the invention to provide an image forming apparatus solving the above problems and being able to include, within the sheet passing area, the arrangement area of the thermal protector whenever a series of printing operations is completed.
According to an aspect of the invented image forming apparatus, because a part or all of overheat preventive devices for preventing the heater from overly heated (thermal protector or protectors) are placed at an area between any two planes which are perpendicular to a conveyance surface of the recording material and are extending parallel in the conveyance direction, through which plural recording material sensors pass, the position of the thermal protector is not subject to printing as a non sheet passing area, thereby improving the reliability of the thermal protector. The margins in respect to the operation standard temperature of the thermal protector are prevented from lowering due to increased temperature of the non sheet passing area caused by printing in a state that the arrangement area of the thermal protector is used as the non sheet passing area.
In another aspect, an image forming apparatus including a fixing device having, for fixing a transferred toner image onto a recording material, a heater for heating the recording material, and an overheating prevention device for the heater, and a recording material sensor such as a recording material existence sensor or a recording material passage sensor on a conveyance route of the recording material, is characterized in that an overheat preventive device or overheat preventive devices for preventing the heater from overly heated is placed astride one plane of planes parallel to the conveyance direction but perpendicular to the conveyance surface of the recording material in passing through any one of the recording material sensors.
According to this image forming apparatus, since the overheat preventive device or overheat preventive devices for preventing the heater from overly heated are placed astride one plane of planes parallel to the conveyance direction but perpendicular to the conveyance surface of the recording material in passing through any one of the recording material sensors, the position of the thermal protector is not subject to printing as a non sheet passing area, thereby improving the reliability of the thermal protector. The margins in respect to the operation standard temperature of the thermal protector are prevented from lowering due to increased temperature of the non sheet passing area caused by printing in a state that the arrangement area of the thermal protector is used as the non sheet passing area.
In the above image forming apparatuses, the fixing device can be of a so-called on-demand type fixing apparatus including a heater for heating a recording material through a thin film and a pressing body for pressing the body to the heater via the thin film.
According to this structure, the image forming apparatus can be provided with not only that the margins in respect to the operation standard temperature of the thermal protector are prevented from lowering due to increased temperature of the non sheet passing area caused by printing in a state that the arrangement area of the thermal protector is used as the non sheet passing area, but also that the thermal capacity of the fixing devise is small.